Conventionally, in order to protect a BGA (Ball Grid Array) junction part of a printed circuit board from an impact due to external force, a protection plate member called a bolster plate or stiffener is disposed on the back side of the surface of the printed circuit board on which the BGA package for semiconductor device is provided, and the protection plate member and the printed circuit board are fixed by fastening them firmly by a bolt and the like.
FIG. 1 is a diagram illustrating a state in which a bolster plate is fixed on the back surface of the surface of a printed circuit board on which a BGA package for semiconductor device is provided.
As illustrated in FIG. 1, a bolster plate 1 is formed to be larger than a BGA package 2 by the size of the fastening positions provided on the four corners. An embedded bolt 3 is set upright on the fastening position on the four corners, and a bolt through-hole is made in advance at the position corresponding to the embedded bolt 3.
FIG. 1 illustrates a state in which the embedded bolt 3 of the bolster plate 1 is put through a bolt through-hole on a printed circuit board 4, and the printed circuit board 4 and the bolter plate 1 is fixed by threadably mounting the lower-end part of a spring pressing stick-like nut 6 held by a heat sink on the head part of the embedded bolt 3 and fastening the spring pressing stick-like nut 6.
FIG. 2 is a diagram illustrating a state in which a stiffener is fixed on the back side of the surface of a printed circuit board on which a BGA package is provided. A stiffener 7 illustrated in FIG. 2 is also formed to be larger than the BGA package 2 by the size of the fastening positions provided on the four corners. A female screw is provided on the fastening position on the four corners.
In addition, a bolt through-hole is made in advance at the position corresponding to the female screw on the printed circuit board 4.
FIG. 2 illustrates a state in which a male screw 8 is put through a bolt through-hole on the printed circuit board 4, and the printed circuit board 4 and the stiffener 7 are fixed by turning a straight slot or a cross slot formed on the head part of the male screw 8 using a straight slot screw driver or a cross slot screw driver to fasten the male screw 8.
Meanwhile, on the printed circuit board 4, a deflection or warpage of the circuit board easily occurs due to the mounting of the BGA package 2.
FIG. 3 is a diagram illustrating a state immediately before fixing a bolster plate on the back side of the surface of a conventional printed circuit board on which a BGA package is provided. As illustrated in FIG. 3, a deflection (or warpage) is generated on the printed circuit board 4 on which the BGA package 2 is mounted.
Therefore, at the stage where the embedded bolt 3 of the bolster plate 1 is put through the bolt through-hole on the printed circuit board 4 and the lower-end part of the spring pressing stick-like nut 6 held by the heat sink 5 is merely mounted on the head part of the embedded bolt 3 threadably, a gap 9 is formed between the upper surface of the bolster plate 1 and the lower surface of the printed circuit board 4.
As the spring pressing stick-like nut 6 is fastened in this state as illustrated with an arrow 10 in FIG. 3, the deflection or warpage generated on the printed circuit board 4 is corrected by the bolster plate 1, and the printed circuit board 4 closely contacts the bolster plate 1 with no gap.
FIG. 4 is a diagram illustrating a state in which the deflection or warpage generated on the printed circuit board 4 as illustrated in FIG. 3 is corrected by fixing a bolster plate on the back side of the surface of the conventional printed circuit board 4 on which the BGA package is provided.
However, the correction generates an internal stress on the printed circuit board 4, and a large strain occurs on the junction part the BGA package 2 to a bump 11, especially on the bump junction part on the outer circumference of the BGA package 2.
FIG. 4 schematically illustrates a strain 12 generated on the junction part of the bump 11. When the large strain 12 is generated on the bump junction part like this, it contributes to the acceleration of fatigue breakdown of the junction part, decreasing the reliability of the junction part and causing a problem that the reliability of the whole device decreases due to the malfunction of an operational circuit device such as a CPU (central processing unit) contained in the BGA package 2.
In addition, the problem occurs in the same way when the stiffener 7 is fixed on the printed circuit board 4 as illustrated in FIG. 2.    Patent document 1: No prior-art document that is comparable to the present invention has been found.